A vehicle mounting a power train referred to as a hybrid system, in which an engine (possibly a known mechanism such as a gasoline engine or a diesel engine) and a motor are combined, has been developed and commercially available. Further, a vehicle (electric vehicle, fuel cell electric vehicle) mounting a power train using only the motor as the running source and not mounting any engine, has also been developed. Such a vehicle mounts an electric storage mechanism (a battery or a capacitor) for motor drive. At the time of acceleration of the vehicle, the motor is driven by the electric power supplied from the electric storage mechanism, to increase the vehicle speed. At the time of braking of the vehicle, the motor functions as a generator, and the generated regenerative energy is stored in the electric storage mechanism. Thus, the kinetic energy of the vehicle is recovered as electric energy, and the regenerative braking force acts on the vehicle. In a vehicle including a battery and a capacitor as electric storage mechanism, at the time of acceleration of the vehicle speed requiring instantaneous high output, it is preferred to supply electric power from the capacitor to the motor, as the capacitor has superior instantaneous output characteristic to the battery. If the capacitor is not sufficiently charged at the time of acceleration, however, it is impossible to supply sufficient electric power from the capacitor to the motor. Japanese Patent Laying-Open No. 05-030608 discloses a technique of charging the capacitor to be ready for vehicle acceleration, in an electric vehicle including a battery and a capacitor.
The hybrid system of electric vehicle disclosed in Japanese Patent Laying-Open No. 05-030608 is applied to an electric vehicle having a DC power source, a motor attached to a wheel, a converter connected to the DC power source and driving the motor, and a controller controlling the converter. The hybrid system includes a battery and a capacitor (a condenser of large capacity) as DC power sources, a switching unit for switching charging/discharging of the battery and the capacitor dependent on acceleration/deceleration, and a limiting unit limiting charging to the battery in a charging mode with regenerative braking, to increase the burden of rapid discharge/charge shared by the capacitor.
According to the hybrid system of electric vehicle disclosed in Japanese Patent Laying-Open No. 05-030608, in the charge mode with regenerative braking, the switching unit is controlled such that the burden of rapid discharge/charge shared by the capacitor having instantaneous output characteristic better than the battery is increased. Thus, rapid charging of the battery is avoided to reduce deterioration of the battery and, in addition, it becomes possible to charge the capacitor beforehand to be ready to supply power from the capacitor at the time of vehicle acceleration when instantaneous high output is required. Therefore, at the time of acceleration, it is possible to increase the vehicle speed with good response.
In the hybrid system for an electric vehicle disclosed in Japanese Patent Laying-Open No. 05-030608, however, it is not always possible to recover maximum regenerative energy. Specifically, if the regenerative energy is stored in the capacitor, the amount of charges to the battery is limited. In some situations in which regenerative energy is large, however, it is possible to charge the battery without limiting the charging amount and to fully charge the capacitor with extra electric power that cannot be stored in the battery any more. If the charging amount to the battery is limited in such a case, the amount of charges to the battery would decrease though the amount of charges to the capacitor is the same, and full recovery of regenerative energy becomes impossible.